This application is related to Japanese Patent Application No. 2001-152195 filed on May 22, 2001 whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a magnetron. More specifically, it relates to a highly reliable magnetron used in microwave heating devices such as microwave ovens or radars. The magnetron inhibits heat generation by an inductor which constitutes a noise-suppressing filter circuit provided on the input side of the magnetron and prevents reverse heating of a cathode of the magnetron.
2. Description of Related Art
Microwave ovens, which are one of microwave devices, are widely used throughout the world. When noise is caused by the microwave oven, it generates noise in radios, television sets and other telecommunication equipment and their normal operation may be hindered. Therefore, the noise from the microwave oven needs to be prevented. The noise from the microwave oven is mainly generated by a magnetron used as a source of microwave oscillation. The noise caused by the magnetron widely ranges from a low-frequency band of several hundred kHz to a high-frequency band of several dozen GHz.
In a conventional magnetron in which a cathode is arranged in the center of an anode tube and an anode cavity is formed around the cathode, a low-pass filter is utilized as one of means of preventing the noise. As shown in FIG. 7 to FIG. 9, the low-pass filter is comprised of an inductor 54 which is connected to cathode terminals 52 and 53 of a magnetron main body 51 to constitute a noise-suppressing filter circuit on the input side (hereinafter referred to as a cored inductor), and a feed-through capacitor 56. The cathode terminals 52 and 53, the cored inductor 54 and the feed-through capacitor 56 are shielded in a filter box 57. The technique of preventing the noise by using the low-pass filter is most commonly adopted to prevent the noise caused by the magnetron in the microwave oven.
The cored inductor 54 is a radio wave absorber and comprised of a core 54b made of ferrite having high relative permeability and a coil 54a made of a cupper wire coated with an insulating material such as polyamide imide and wound about the outer circumference of the core 54b so that turns thereof are in close contact. The cored inductor 54 is connected to the cathode terminals 52 and 53 with the intervention of an electrical linear portion 54c. The length of the linear portion 54c is adjusted so that impedance on the cathode terminals 52 and 53 as observed from the cathode will be infinitely large. From the viewpoint of designing the magnetron, the length of the linear portion 54c is one of important factors in order to prevent a fundamental frequency of the microwave induced by the cathode (an oscillated frequency, e.g., a microwave of 2,450 MHz) from leaking out of the cathode terminals 52 and 53. Therefore, the length of the linear portion 54c is optimally determined in accordance with the design of the magnetron main body 51.
If the fundamental oscillation frequency generated in the magnetron main body 51, for example, part of a microwave output of 2,450 MHz, is leaked into the cathode terminals 52 and 53 together with the noise, the oscillated microwave is wasted and the core 54b absorbs the microwave energy. As a result, the oscillation efficiency decreases. Further, if large microwave energy is leaked out, the core 54b generates heat and an insulative coating on the coil 54a is burned out to cause dielectric breakdown, or the temperature of the feed-through capacitor 56 connected in series is raised to cause dielectric breakdown. Therefore, the length of the linear portion 54c is adjusted so that the impedance on the cathode terminals 52 and 53 as observed from the cathode will be the maximum to reduce the leakage of the microwave energy.
Japanese Patent Publication No. 57(1982)-17344 describes a technique of reflecting or attenuating the leaked microwave energy by connecting between the cored inductor and the cathode terminals an air-core inductor which does not have the ferrite core.
However, in view of characteristics of the magnetron, a phenomenon called reverse heating of the cathode should be considered. Among electrons rotating between the anode tube and the cathode of the magnetron, generated are electrons which are accelerated by a high-frequency electric field and then collide against the cathode. The reverse heating of the cathode is a phenomenon in which a filament to constitute the cathode is heated and damaged by the collision of the electrons. If load impedance increases due to the reverse heating of the cathode, life of the filament may be extremely shortened.
The reverse heating of the cathode can also be optimized by adjusting the length of the linear portion 54c extending from the cathode terminals 52 and 53 to the cored inductor. However, as shown in FIG. 10, the optimum length with respect to the reverse heating of the cathode and that with respect to the temperature variation of the cored inductor do not agree with each other. FIG. 10 shows the temperature variation of the cored inductor 54 (solid line R1) and the reverse heating of the cathode of the magnetron (broken line R2) with respect to the length of the linear portion 54c of the conventional magnetron, respectively. The reverse heating of the cathode of the magnetron is shown by the ratio of filament currents before and after the oscillation expressed as a percentage. The smaller the value is, the greater influence is caused by the reverse heating of the cathode.
Even in the case where the air-core inductor without the core is connected between the cored inductor and the cathode terminals as described in Japanese Patent Publication No. 57(1982)-17344, the length of the linear portion extending from the air-core inductor to the cathode terminals needs to be optimized. Therefore, the design of the inductor is complicated and the size of the whole inductor may increase. Accordingly, an insulating distance between the inductor and the filter box needs to be ensured, which inevitably increases the size of the filter box as well.
The present invention has been in view of the above-described circumstances. An object of the present invention is to provide a highly reliable magnetron capable of inhibiting heat generation by the inductor which constitutes the noise-suppressing filter circuit provided on the input side of the magnetron and preventing the reverse heating of the cathode of the magnetron, without increasing the size of the filter box.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter.
In order to achieve the above object, according to the present invention, there is provided a magnetron comprising a cathode terminal of a magnetron main body and an inductor connected to the cathode terminal to constitute a filter, wherein the inductor includes an air-core coarse inductor and a cored inductor connected in series, the air-core coarse inductor being connected to the cathode terminal side and the air-core coarse inductor includes a large pitch winding (hereinafter referred to as a large pitch winding) provided on the cathode terminal side and a small pitch winding (hereinafter referred to as a small pitch winding) provided on the opposite side.
In the invention, it is preferred that an interval between the air-core coarse inductor and the cored inductor connected in series is 3.0 mm or more. It is also preferred that an interval between turns of the small pitch winding is 1.0 mm or less. Further, it is preferred that an interval between turns of the large pitch winding is 1.5 mm or more. Still Further, it is preferred that the number of turns of the small pitch winding is 1.5 or more.
However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.